What Constraint Can Only Be Applied at Table Level or With an Alter Statement?

SQL - Constraints

Constraints are the rules enforced on the data columns of a table. These are used to limit the type of data that tin can go into a table. This ensures the accuracy and reliability of the data in the database.

Constraints could exist either on a cavalcade level or a table level. The cavalcade level constraints are practical only to i cavalcade, whereas the table level constraints are practical to the whole table.

Following are some of the near usually used constraints available in SQL. These constraints have already been discussed in SQL - RDBMS Concepts affiliate, but it'south worth to revise them at this bespeak.

NOT Cipher Constraint − Ensures that a cavalcade cannot have Zip value.
DEFAULT Constraint − Provides a default value for a cavalcade when none is specified.
UNIQUE Constraint − Ensures that all values in a column are unlike.
Chief Primal − Uniquely identifies each row/tape in a database table.
Foreign Key − Uniquely identifies a row/record in any of the given database table.
CHECK Constraint − The CHECK constraint ensures that all the values in a column satisfies certain atmospheric condition.
Index − Used to create and retrieve data from the database very quickly.

Constraints can be specified when a table is created with the CREATE Table statement or you tin can apply the Alter TABLE statement to create constraints fifty-fifty after the table is created.

Dropping Constraints

Any constraint that you have defined can be dropped using the Modify TABLE control with the DROP CONSTRAINT option.

For example, to drop the main key constraint in the EMPLOYEES table, yous tin can use the following command.

ALTER Table EMPLOYEES Driblet CONSTRAINT EMPLOYEES_PK;

Some implementations may provide shortcuts for dropping certain constraints. For case, to drop the main primal constraint for a tabular array in Oracle, y'all can use the following command.

Change TABLE EMPLOYEES Driblet Master KEY;

Some implementations allow you to disable constraints. Instead of permanently dropping a constraint from the database, you may want to temporarily disable the constraint and then enable it later.

Integrity Constraints

Integrity constraints are used to ensure accuracy and consistency of the information in a relational database. Data integrity is handled in a relational database through the concept of referential integrity.

There are many types of integrity constraints that play a role in Referential Integrity (RI). These constraints include Primary Key, Foreign Cardinal, Unique Constraints and other constraints which are mentioned above.